In brew kettle operation, heating of the wort causes coagulation of proteins, sterilization of the wort and the destruction of enzymes, extraction of hops and evaporation of excess water to provide a general change of the wort through boiling. The evaporation rate, temperature and circulation of wort in the brew kettle are important factors in the effective coagulation of protein as well as hop extraction and hop character.
In the past, "fire brewing" has been used to impart improved flavor characteristics to the beer. In traditional fire brewing, the wort is boiled in small copper kettles that are subjected to direct oil or gas firing. The direct firing of the work in the copper kettles is believed to effect the carmelization and improve the flavor of the beer.
It is also known to circulate boiling wort through external heat exchangers and return the wort to the brew kettle through center tube spreaders in order to obtain more efficient mixing and energy savings. In a system of this type, known as Kalandria, the wort is passed through stainless steel tubes in an external heat exchanger in heat transfer relation to steam, and the wort, having been heated several degrees, is then returned to the brew kettle and is discharged through a center tube above the level of wort in the kettle.